Originally Posted by https://www.easa.europa.eu/the-agency/faqs/brexit#category-aircrew-training-and-licensing-
As of January 1, 2021 licenses and certificates issued by UK will no longer be valid in EASA Member States and will be treated as a third country licences and certificates. Accordingly, as of that date a UK issued ATPL cannot be used to operate aircraft of commercial operators under oversight of EASA Member States.
As of January 1, 2021 in order to obtain a Part-FCL licence from an EASA Member State, UK licence holders must follow a conversion process as per Commission Delegated Regulation (EU) No 2020/723.